Catalytic hydrogenation of the compounds (II) has already been described, but the selectivities for the product corresponding to the removal of a single chlorine atom are low. Thus, hydrogenolysis of 1,1-dichloro-1,2,2,2-tetrafluoroethane at 280.degree. C. over a catalyst containing 5% palladium on charcoal (Patent GB 1,578,933) yields a product containing 70% of 1,1,1,2-tetrafluoroethane. Similar results are obtained by C. Gervasutti et al. (Journal of Fluorine Chemistry 1981, 1, 1-20) over a catalyst containing 0.5% of palladium on charcoal: at 170.degree. C. the hydrogenolysis of 1,1-dichloro-1,2,2,2-tetrafluoroethane produces 76% of 1,1,1,2-tetrafluoroethane.
Charcoal-supported metal catalysts containing 0.5% of rhenium, platinum or rhodium (Publications JP 132,536/89, 132,537/89 and 132,538/89 respectively) have also been tested in the hydrogenolysis of 1,1-dichloro-1,2,2,2-tetrafluoroethane; they result in the significant formation of 1,1,1-trifluoroethane, which can attain 30% at 200.degree. C. Furthermore, the complete dechlorination of 1,1-dichloro-1,2,2,2-tetrafluoroethane on bimetallic catalysts based on palladium (Publication JP 172,348/89) or based on platinum (Publications JP 172,349/89 and 128,942/89) has also been described.
To solve the problem of the removal of a single chlorine atom it is necessary to resort, according to Japanese Patent Application 106,051/82 (Publication JP 222,038/83), to a chemical reduction with the zinc-ethanol couple; under the conditions described, the selectivity of the hydrogenolysis of 1,1,1-trichloro-2,2,2-trifluoroethane to 1,1-dichloro-2,2,2-trifluoroethane attains 90%. However, this process has the disadvantage of employing costly metallic zinc and of giving zinc chloride as a byproduct, which must be destroyed.
Advantageous results have been obtained in the catalytic hydrogenolysis with catalysts containing 0.5% of platinum on alumina or charcoal (Publication JP 149,739/89): at 175.degree. C., with a catalyst on charcoal, 1,1,1-trichloro-2,2,2-trifluoroethane yields a product containing 64% of 1,1-dichloro-2,2,2-trifluoroethane, and at 200.degree. C., with a catalyst on alumina, 1,1-dichloro-1,2,2,2-tetrafluoroethane yields a product containing 42% of 1-chloro-1,2,2,2-tetrafluoroethane.